“Stonewall,” Roland Emmerich’s would-be epic film about a turning point in the gay liberation movement in 1969, is far from the first historical movie to choke on its own noble intentions. For its two-hour-plus duration, the movie struggles to fuse incompatible concepts.

On one level, “Stonewall” is a sweeping social allegory whose central character, Danny Winters (Jeremy Irvine of “War Horse”), is an all-American boy from the provinces (Indiana) thrown out of the house by his father (David Cubitt), a high school football coach, for being gay. Arriving in New York with little money and no fixed abode, Danny is radicalized by observing, then experiencing, police brutality.

On another level, the movie wants to be as specific as possible in its reconstruction of chaotic events that took place 46 years ago and have acquired a mythic dimension that demands heroic enlargement. In hindsight, the Stonewall riots are rather like the Woodstock music festival later that summer. More people claim to have been present than could possibly have been there. But except for its identification of actual police officers, “Stonewall” doesn’t bother to distinguish among facts, fiction and urban legend.

Early scenes jump between Danny’s final days in Indiana, before he is observed having sex in a car with a high school quarterback, and his new life in New York. Exiled from his biological family, he bonds with a group of outsiders, homeless drag queens and hustlers who live on the streets or pile, as many as a dozen at time, into a shabby apartment. The neighborhood center of gravity is the seedy mob-owned Stonewall Inn, which is subject to periodic police raids.

The movie, filmed in Montreal, does a reasonably good job of evoking the heady mixture of wildness and dread that permeated Greenwich Village street life in those days. In the summer of ’69, homosexual behavior between consenting adults was illegal in New York. At any moment, the police could descend on a gay bar, round up the customers and haul them off.

By many accounts, the rebellion was led by drag queens and gay street people who for the first time stood up to the police, and “Stonewall” dutifully acknowledges their participation. But, its invention of a generic white knight who prompted the riots by hurling the first brick into a window is tantamount to stealing history from the people who made it. A trailer that focuses on that moment has led some gay activists to threaten a boycott of the film. No matter how much Mr. Emmerich and Jon Robin Baitz, the estimable playwright who wrote the screenplay, insist that the movie pays tribute to a full multiethnic range of gay and lesbian characters, “Stonewall” falls short. Like it or not, symbolism matters.

Had the movie’s central character been Ray, a.k.a. Ramona (Jonny Beauchamp), an androgynous, volatile Puerto Rican who unrequitedly falls in love with Danny, there might be no quarrel. Ray’s saucy “girlfriends” include characters with nicknames like Queen Cong (Vladimir Alexis) and Little Orphan Annie (Caleb Landry Jones), who are treated with respect but remain peripheral.

The feisty, lovelorn Ray is far and away the strongest, most complex character, and Mr. Beauchamp gives him his due, even though too many of his speeches sound like a mix of biographical filler and boilerplate sloganeering.

The protests stretched over four nights in 1969 but are condensed into one night in the film, and the severity of the violence in scenes of police harassment during the rebellion makes your blood boil. But in these overwrought scenes, Mr. Emmerich has reverted to the hyperadrenalized style of his big-budget disaster films like “The Day After Tomorrow” and “Independence Day.”

The movie goes off on weird tangents, of which the most unnecessary and exploitative is a scene in which Danny is forcibly pimped out by Ed Murphy (Ron Perlman), one of the bar’s real-life managers (who later became a respected gay-rights advocate), to a cross-dressing gargoyle who suggests a nightmare fantasy of J. Edgar Hoover. A terminally sullen Jonathan Rhys Meyers plays Danny’s first New York crush, Trevor, a leader of the Mattachine Society, the seminal gay-rights organization.

The strident tone of Mr. Baitz’s screenplay is so far removed from the subtlety and sophistication of his plays, like “Other Desert Cities,” and television shows, like “Brothers and Sisters,” that the most logical explanation for its bluntness is that he felt compelled to substitute strident melodrama and agitprop for psychological complexity. Too much of his dialogue gives “Stonewall” the self-conscious tone of an inspirational teaching tool for future generations of young people.